Atmospheric Chemistry

Sasha MadronichNational Center for Atmospheric ResearchBoulder, Colorado USA

Boulder, 1 June 2009

Earth’s Atmosphere

Composition• 78% nitrogen• 21% oxygen• 1-2% water (gas, liquid, ice)• trace amounts (<< 1%) of many other species, some natural

and some “pollutants”

Reactivity dominated by • oxygen chemistry• solar photons

To understand fate of pollutants, must first understand oxygen photochemistry

2

Pure Oxygen Species

3

Energetics of Oxygen in the Atmosphere

DHf (298K) kcal mol-1

Excited atoms O*(1D) 104.9

Ground state atoms O (3P) 59.6

Ozone O3 34.1

“Normal” molecules O2 0

4

Increasingstability

Atmospheric OxygenThermodynamic vs. Actual

1E-110

1E-100

1E-90

1E-80

1E-70

1E-60

1E-50

1E-40

1E-30

1E-20

1E-10

1

200 220 240 260 280 300

Temperature, K

Con

cent

ratio

n, a

tm. O2 (=0.21)

thermodyn. O3thermodyn. Othermodyn. O*observed O3inferred Oinferred O*

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O3

O

O*

Photochemistry

Thermodynamics alone cannot explain atmospheric amounts of O3, O, O*

Need – energy input, e.g.

O2 + hn O + O (l < 242 nm)

– chemical reactions, e.g. O + O2 (+ M) O3 (+ M)

= Photochemistry

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Stratospheric Ozone Chemistry

• The Only Production: O2 + hn (l < 242 nm) O + O

Chapman 1930 O + O2 + M O3 + M

• Several Destruction Reactions:Pure oxygen chemistry: O3 + hn (l < 800 nm) O +

O2

Chapman 1930 O + O3 2 O2

Catalytic Cycles:

Odd hydrogen (HOx = OH + HO2) O3 + OH O2 + HO2

Bates and Nicolet 1950 O + HO2 O2 + OHO3 + HO2 2 O2 + OH

Odd nitrogen (NOx = NO + NO2) O3 + NO O2 + NO2

Crutzen 1970 O + NO2 O2 + NO

Halogens (Cl, Br) O3 + Cl O2 + ClORowland and Molina 1974 O + ClO O2 + Cl

8

COLUMN OZONE TRENDS, %

9http://www.cpc.ncep.noaa.gov/products/stratosphere/winter_bulletins/sh_07/Fig_5.gif

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SOLAR SPECTRUM

UNEP, 2002

O2 and O3 absorball UV-C (l<280 nm)before it reaches the troposphere

Tropospheric Ozone Formation - How?

Laboratory studies show that O3 is made almost exclusively by the reaction:

O2 + O + M O3 + M

But no tropospheric UV-C radiation to break O2

O2 + hn (l < 242 nm) O + O

Haagen-Smit(1950s) - Los Angeles smog: Urban ozone (O3) is generated when air containing hydrocarbons and nitrogen oxides (NOx = NO + NO2) is exposed to tropospheric UV radiation

The Nitrogen Family

N nitrogen atoms – negligible at room TN2 molecular nitrogen

Zeldovich mechanism at high T (flames, engines, lightning):O2 + heat O + O

O + N2 N + NON + O2 O + NO

(NO is the cross-product of scrambling N2 and O2 at high T)

Nitrogen oxides : NOx ≡ NO + NO2

NO nitric oxide is 90-95% of direct emissions NO2 nitrogen dioxide is 5-10% of direct emissions, but

more is made from NO + oxidants in the atmosphere 12

(some other nitrogen species)

NO3 nitrate radicalN2O5 dinitrogen tetroxide

HONO nitrous acidHONO2 nitric acidCH3ONO2 methyl nitrate

N2O nitrous oxide (laughing gas)

NH3 ammoniaNH2CH3 methyl amine

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Tropospheric O3 Formation - 2

NO2 photo-dissociation is the source of O atoms that make tropospheric O3

NO2 + hn (l < 420 nm) NO + O

O + O2 + M O3 + M

_____________________________________________

Net: NO2 + hn + O2 NO + O3

CALCULATION OF PHOTODISSOCIATION COEFFICIENTS

J (s-1) = l F(l) s(l) f(l) dl

F(l) = spectral actinic flux, quanta cm-2 s-1 nm-1

probability of photon near molecule.

s(l) = absorption cross section, cm2 molec-1

probability that photon is absorbed.

f(l) = photodissociation quantum yield, molec quanta-1

probability that absorbed photon causes dissociation.

NO2 + hn (l < 420 nm) NO + O

16

Mexico City, surface, March 2006

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Tropospheric O3 Formation - 3

NO2 photo-dissociation makes some O3, but not enough. Two problems:

Usually O3 ~ 20 - 500 ppb >> NO2 ~ 1 – 10 ppb

Reversal by the reaction:NO + O3 NO2 + O2

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Tropospheric O3 Formation - 4

Initiation by UV radiation (Levy, 1970):O3 + hn (l < 330 nm) O*(1D) + O2

O*(1D) + H2O OH + OH

Hydrocarbon consumption (oxygen entry point):OH + RH R + H2OR + O2 + M ROO + M

Single-bonded oxygen transferred to NOx:ROO + NO RO + NO2

NOx gives up oxygen atoms (as before):NO2 + hn (l < 420 nm) NO + OO + O2 + M O3 + M

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Tropospheric O3 Formation - 5

PropagationRO + O2 R’CO + HOOHOO + NO OH + NO2

every NO NO2 conversion makes O3

except NO + O3 NO2 + O2

Termination OH + NO2 + M HNO3 + M HOO + HOO + M H2O2 + M HOO + O3 OH + 2 O2

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Initiation by photo-dissociationO3 + hn + H2O 2 OH + O2

Oxidation of hydrocarbonsOH + RH + O2 + M ROO + H2O + M

NO NO2 conversionsROO + NO RO + NO2

O3 + NO NO2 + O2

Actual O3 formationNO2 + hn + O2 O3 + NO

PropagationRO + O2 HOO + R’COHOO + NO OH + NO2

TerminationOH + NO2 + M HNO3 + MHOO + HOO + M H2O2 + O2 + MHOO + O3 OH + 2 O2

Summary of Key Steps In Tropospheric O3 Formation

NOx Photo-stationary State

NO2 + hn NO + O3 JNO2

NO + O3 NO2 + O2 k1

NO + HO2 NO2 + OH k2

NO + RO2 NO2 + RO k3

d[NO]/dt = +JNO2[NO2] – [NO](k1[O3]+k2[HO2]+k3[RO2])

~ 0 at steady state

f ≡ JNO2[NO2] / (k1[NO][O3]) ~ 1 + (k2[HO2]+k3[RO2]) /k1[O3]

Can use measurements of f to estimate [HO2] + [RO2] and instantaneous O3 production

21

DIURNAL AND WEEKLY VARIATIONSSurface network in Mexico City

22Stephens et al., ACP 2008

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Tropospheric Chemical Mechanisms

This talk: 10 reactions

Typical 3D model used for air quality: 100 - 200 reactions

Typical 0D (box) models used for sensitivity studies:5,000 - 10,000 reactions

Fully explicit (computer-generated) mechanisms:106 - 107 reactions

Atmospheric Volatile Organic Compounds (VOCs): Hydrocarbons

AlkanesCH4

CH3CH3

CH3CH2CH3

C4H10 (2 isomers)C5H12 (3 isomers)C6H14 (5 isomers)C7H16 (9 isomers)C8H18 (18 isomers)….

methaneethanepropanebutanepentanehexaneheptaneoctane….

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Atmospheric VOC’s: Hydrocarbons - 2

AlkenesCH2=CH2

CH2=CHCH3

…CH2=C(CH3)CH=CH3

AromaticsC6H6

C6H5CH3

C6H5(CH3)2 (3 isomers)

… Terpenes

C10H16

ethene (ethylene)propene (propylene)…2-methyl 1,3 butadiene

(isoprene)

benzenetoluenexylenes…

a-pinine, b-pinine…

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Global Hydrocarbon Emissions

Tg C yr-1

Isoprene Terpenes C2H6 C3H8 C4H10 C2H4 C3H6 C2H2 Benzene Toluene

Fossil fuel - - 4.8 4.9 8.3 8.6 8.6 2.3 4.6 13.7

Biomass burning

- - 5.6 3.3 1.7 8.6 4.3 1.8 2.8 1.8

Vegetation 503 123 4.0 4.1 2.5 8.6 8.6 - - -

Oceans - - 0.8 1.1 - 1.6 1.4 - - -

TOTAL 503 123 15.2 13.4 12.5 27.4 22.9 4.1 7.4 15.5

27Ehhalt, 1999

CH4 ~ 500 – 600 Tg CH4 yr-1 [IPCC, 2001]

Atmospheric VOC’s:Substituted Hydrocarbons

Alcohols, -OH– methanol, CH3OH – ethanol, CH3CH2OH

Aldehydes, -CHO– formaldehyde,CH2O– acetaldehyde, CH3CHO

Ketones, -CO-– acetone, CH3COCH3– MEK, CH3COCH2CH3

Carboxylic acids, -CO(OH)– formic, HCO(OH)– acetic, CH3CO(OOH)

Organic hydroperoxides, -OOH– methyl hydroperoxide, CH3(OOH)

Organic peroxy acids, -CO(OOH)– peracetic, CH3CO(OOH)

Organic nitrates, -ONO2– methyl nitrate, CH3(ONO2)– Ethyl nitrate, CH3CH2(ONO2)

Peroxy nitrates, -OONO2– methyl peroxy nitrate,

CH3(OONO2)

Acyl peroxy nitrates, -CO(OONO2)– PAN, CH3CO(OONO2)

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Atmospheric Organic Radicals

Alkyl (carbon-centered)CH3 methylCH2CH3 ethylCH2CH2CH3 propyl

Peroxy, -OOCH3OO methyl peroxyCH3CH2OO ethyl peroxy

Alkoxy, -OCH3O methoxyCH3CH2O

ethoxy

Acyl, CO(OO)CH3CO(OO) acetyl

Criegee, C(OO)CH2OO from O3 + C2H4

CH3CHOO from O3 + C3H6

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General Hydrocarbon Reaction Patterns

Short-chain compounds tend to have unique behavior, and must be considered individually.

Longer-chain compounds are quite alike within each family (e.g. all aldehydes). Kinetics and mechanisms can be adjusted for chain length and substitutions (structure-activity relations).

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31

RH

R

ROO

RO

R’CHO

CO2 + H2O

ROOHRONO2

…

OH, O3, NO3

O2

NO

NO HO2

hnhn

O2, heat

OH, O3, NO3

OH

OH

OH

Generalized OxidationSequence of Hydrocarbons

OH + Hydrocarbon Reactions

Abstraction of HOH + CH3CH3 CH3CH2

…followed immediately byCH3CH2 + O2 + M CH3CH2OO + M

Addition to double bondsOH + CH2=CH2 CH2(OH)CH2

…followed immediately byCH2(OH)CH2+ O2 + M CH3(OH)CH2OO + M

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O3 + Hydrocarbon Reactions

Ozone addition across double bond

O3 + CH2=CH2 CH2 – CH2 CH2O + (CH2OO)*

Fate of excited Criegee diradical:(CH2OO)* CO + H2O

CO2 + H2

CO2 + 2 H …

+ M CH2OO (stabilized Criegee diradical)

CH2OO + (H2O, NO, NO2, SO2) Products

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O O O

NO3 + VOC Reactions

H atom abstraction:CH3CHO + NO3 CH3CO + HNO3

CH3CO + O2 + M CH3CO(OO) + M

Addition to double bond:CH2=CH2 + NO3 + M CH2(ONO2)CH2 + MCH2(ONO2)CH2 + O2 + M CH2(ONO2)CH2(OO) + M

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Peroxy Radical Reactions - 1

with NOROO + NO RO + NO2

ROO + NO + M RONO2 + M

with NO2

ROO + NO2 + M ROONO2 + MRCO(OO) + NO2 + M RCO(OONO2) + M

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Peroxy Radical Reactions - 2

with HO2

ROO + HOO ROOH + O2

RCO(OO) + HOO RCO(OOH) + O2

with other organic peroxy radicals, e.g.CH3CH2OO + CH3OO

radical channel CH3CH2O + CH3O + O2

molecular channel 1 CH3CH2OH + CH2O + O2

molecular channel 2 CH3CHO + CH3OH + O2

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Alkoxy Radical Reactions

with O2, e.g.CH3CH2O + O2 CH3CHO + HOO CH3CH(O)CH3 + O2 CH3COCH3 + HOO

thermal decomposition, e.g.CH2CH(O)CH2OH + M CH3CHO + CH2OH + M

isomerization, e.g.CH3CH(O)CH2CH2CH2CH3 CH3CH(OH)CH2CH2 CHCH3

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Reactions of Partly Oxidized Species

OH, O3, and NO3 reactions as with precursor hydrocarbons

photolysis important for– aldehydes– ketones– peroxides– alkyl nitrates– but not for alcohols or carboxylic acids

thermal decomposition for peroxy nitrates

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Simplified Mechanism for Pentane (C5H12)

Multiple NONO2 conversionsproduce O3

Organic nitrates allow long-rangetransport of NOx

Radical sinks:Some are temporary, producing HOx later

Some have low vapor pressures,can make organic aerosols

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Consequences of tropospheric chemistry - 1

Formation of O3

Urban: 100-500 ppbRegional: 50-100 ppbGlobal background increase

10-20 ppb 35-45 ppb in NH10-20 ppb 25-35 ppb in SH

Damage to health and vegetatione.g. $3.5B-6.1B /yr in US for 8 major crops (Murphy, 1999)

Greenhouse role of O3

Changes in global oxidation capacity

EPA, 2004

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Consequences of tropospheric chemistry - 2

Formation of peroxides and acids:

HO2 + HO2 H2O2 + O2

OH + NO2 + M HNO3 + MOH + SO2 … H2SO4

H2O2(aq) + SO2(aq) … H2SO4(aq)

Damage to vegetation, lakes, and buildings (acid precipitation)

Sulfate aerosol formation (visibility, precipitation, direct and indirect radiative forcing of climate)

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Consequences of tropospheric chemistry - 3

Products of hydrocarbon oxidationCO2 (minor compared to direct emissions)CO (~ 1/3 of total emissions)Oxygenated organics: aldehydes, ketones, alcohols,

organic acids, nitrates, peroxides

Damage to health, vegetation Secondary organic aerosol formation (health,

visibility, meteorology, direct and indirect climate forcing)

Changes in global oxidation capacity

Organic aerosol > Sulfate in most observations

Zhang et al., GRL 2007

Human Health Impacts of Particles

For 2002 (World Health Organization, 2007):

• World: 865,000 deaths per year 1.0 DALY* /1000 capita per year

* DALY = Disability-Adjusted Lost Years

• U.S.: 41,200 deaths per year 0.8 DALY /1000 capita per

year

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Global Oxidation (self-cleaning) Capacity

Solar UV radiation

Oxidation, e.g.:

CH4 + OH … CO2 + H2O

Insoluble Soluble

EmissionsCH4 CmHn

SO2

NO

CO

NO2

HalocarbonsDeposition(dry, wet)

HNO3, NO3-

H2SO4, SO4=

HCl, Cl-

Carboxylic acids

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Consequences of tropospheric chemistry - 4

Oxidizing Capacity:• Increase because of increasing emissions of NOx?• Increase because of increasing UV radiation?

or• Decrease because of increasing emissions of CO, CmHn,

SO2, and other reduced compounds?

Decreased OH (oxidizing capacity) implies generally higher amounts of most pollutants including:• Higher amounts of greenhouse gases• Higher amounts of substances that deplete the ozone layer• More global spread

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TROPOSPHERIC OXIDIZING (SELF-CLEANING) CAPACITYLog10 [OH] - Global Box Model

Different OH regimes

106

106

105

105

104

104

103

103

102

102 107

101

100

FCH4, cm-3 s-1

F NO, c

m-3 s

-1

FO3=5e4 cm-3 s-1, FCO=1e5 cm-3 s-1

~current

Madronich and Hess, 1993

pre-industrial

future?

FUTURE TROPOSPHERIC O3: MODELS DISAGREE

IPCC 2001

SUMMARY

• Stratospheric chemistry is relatively simple:• Oxygen photo-dissociation • Ozone catalytic destruction• Impacts: climate, surface UV radiation

• Tropospheric chemistry is complex, non-linear:• Ozone made from UV, NOx, and HCs• Many hydrocarbons (biogenic and anthropogenic)• Aerosols: most are made in atmosphere by condensation

of gas phase species• Many impacts: health, ecosystems, meteorology, climate

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